Cooling system for lasing media

ABSTRACT

Heat generated within a lasing medium operating in a laser pumping cavity is efficiently removed from the pumping cavity by use of a heat sink located exterior of the cavity. A tube formed of a single crystal of sapphire surrounds the laser medium and extends through an end wall of the pumping cavity. There, the sapphire tube is in turn surrounded by a metal heat exchanger element that may be operated at cryogenic temperatures. Undesired condensation is prevented from forming adjacent the laser medium by forming the pumping cavity as a sealed chamber and evacuating it to a relatively low gas pressure.

United States Patent [151- 3,676,798 McMahon 1 July 11, 1972 s41 COOLINGSYSTEM FOR LASING OTHER PUBLICATIONS [72] Inventor: Donald H. McMahon,Carlisle, Mass.

[73] Assignee: Sperry Rand Corporation [22] Filed: Oct. 13, 1969 I21 IAppl, No.1 365,910

[52] US. Cl .L ..33l/94.5, 165/185 [51] Int. Cl ..H0ls 3/04, F281 7/00[58] Field of Search ..33 1/945 [56] References Cited UNITED STATESPATENTS 3,387,227 6/1968 Mastrup et al ..331/94.5 3,339,150 8/1967Bowness ..331/94.5

3,339,151 8/1967 Smith ..33l/94.5 3,471,801 10/1969 Woodbury et al......331/94.5 3,361,989 1/1968 Sirons ..331/94.5 3,353,115 11/1967Maiman ..33 1/945 MEDIA PULSE I GENERATOR E AND POWER SUPPLY Holland,Thermal Conductivity Materials," 1. App. Phys. Vol. 33, No. 9, Sept.1962, pp. 2910- 2911.

Thermal Conductivity of Optical Materials" AlP Handbook, 2d. ed. p. 4-94.

Levine, Lasers," Vol. 1, Marcel Delker pub. co. (New York) 1966, pp.137-153, 177-180.

Primary Exuminerwilliam L. Sikes Ass-("slam ExuminerR. J Webster A!mrne\-S. C. Yeaton 57 ABSTRACT Heat generated within a lasing mediumoperating in a laser pumping cavity is efficiently removed from thepumping cavity by use of a heat sink located exterior of the cavity. Atube formed of a single crystal of sapphire surrounds the laser mediumand extends through an end wall of the pumping cavity. There, thesapphire tube is in turn surrounded by a metal heat exchanger elementthat may be operated at cryogenic temperatures. Undesired condensationis prevented from forming adjacent the laser medium by forming thepumping cavity as a sealed chamber and evacuating it to a relatively lowgas pressure.

2 Claims, 3 Drawing Figures Patented July 11, 1972 3,676,798

2 Sheets-Sheet l PULSE GENERATOR 1,

AN D POWER SUPPLY 25 7 x I/Vl/E/VTOR DONALD H. Ma MAHO/V BACKGROUND OFTHE INVENTION l. Field of the Invention The invention pertains to meansfor the generation of electromagnetic energy by stimulated emission ofsuch radiation. A laser medium capable of producing stimulated emissionis disposed in an optical pumping chamber. Whensubjected to the intensepumping energy, the temperature of the lasing medium may increasegreatly if thermal energy is not efficiently removed from it. Therefore,the invention particularly pertains to means for the efficient,convenient removal of thermal energy from the laser medium for disposalexterior of the pumping cavity.

2. Description of the Prior Art One problem that has been experiencedwith the use of solid state lasers operating at relatively high energylevels is that when the laser medium is subjected to the intense energyof a pumping lamp in a confining laser pump cavity, the temperature ofthe laser medium may reach relatively high levels. Efficient removal ofsuch thermal energy is required, especially as a high temperature in thelasing medium may either reduce the efficiency of useful transfer ofenergy within the lasing medium or may even prevent lasing operationentirely.

Certain types of lasers may even require operation at depressedtemperatures. For instance, some types of garnet lasing materialsoperate efficiently in the region of 77 Kelvin, but do not operate atall, or operate only at a low efficiency level at 300 Kelvin (roomtemperature).

Operation of these devices has indeed been demonstrated in the past,though with relatively crude arrangements which lack the attributes ofconvenience and versatility required of commercially acceptableapparatus. For example, garnet and other lasing media have been operatedat cryogenic temperatures in equipment in which a bulky Dewar vessel isplaced actually within the pumping cavity. The inherent size of theDewar vessel has placed severe restrictions on the design of the pumpingcavity and has prevented the supply of compact equipment convenient foruse in many applications.

Furthermore, components of the liquified gas used as the cooling agenttend to recondense on surfaces adjacent the lasing medium. Such isevidently true also of certain components of room air which may bepresent. The use of the Dewar vessel in providing the necessary coolinghas prevented effective control over the undesirable formation ofcondensate adjacent the lasing material. The presence of such condensatealso undesirably limits the efficiency of operation of the lasingmaterial.

SUMMARY OF THE INVENTION The present invention relates to a laser deviceof the type using a crystalline rod of optically active material whoseends are polished, parallel, and coated with a thin film of material forat least partially reflecting light and whose cylindrical sides aretransparent to pump radiation. The latter radiation excites ions in thecrystalline rod to emit radiation which travels along the rod and isamplified as coherent radiation. The substantial amount of pumping powerwhich appears as heat within the lasing medium is removed therefrom byuse of a composite rod structure having a core of laser material coveredby a sheath of transparent optically refractive material. The structurefacilitates cooling by removing thermal energy from the vicinity of thelasing material to a remotely located, cryogenically cooled, metallicsheath forming the core of a second heat exchanger. Heat flow isefficiently carried out by longitudinal flow of energy along thetransparent sheath, rather than merely radially through its surface. Theinvention enables the practical use of a convenient configuration forthe laser system, in which parts primarily concerned with cooling arelocated exterior of the pump cavity and its design is then dictatedprimarily by considerations of what is required for efficientstimulation of emission in the laser material. Furthermore, the pumpcavity may be conveniently evacuated or filled with a suitable inert gasfor preventing interference with the operation of the laser because offormation of condensate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view partly in section ofa laser device embodying the principles of this invention;

FIG. 2 is a sectional view of the elliptical laser device of FIG. 1; and

FIG. 3 is a cross-section view of another form of the inven tion.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there ispresented a laser device ac cording to the present invention whichincludes as principal elements a composite lasing and coolingconfiguration I, a high intensity lamp 2 for stimulating lasing actionin configuration 1, and an optical pumping cavity 3 surrounding aportion of the composite lasing-cooling configuration 1 and the pumpinglamp 2. The wall 5 of the optical cavity 3 is elliptical incross-section, as is shown clearly in FIG. 2. The cavity 3 is defined inpart by an enclosure or casing 4 having the inner elliptically contouredwall 5 and closed at its ends by apertu red flat end walls 6 and 7.

Aperture 8 in end wall 7 provides means for inserting within the cavity3 an emission stimulating or pumping lamp 2. Lamp 2 is illustrated asmounted in a socket 9 supported on a bracket 20 from end wall 7 bysuitable fasteners 21 and 22.

Depending upon the choice of lamp 2, it is supplied with pulsedelectrical energy or any other necessary operating power in a knownmanner via a plurality 25 of electrical leads connecting the lamp socket9 to a pulse generator and power supply 26 which may both be ofconventional nature. Lamp 2 may be, for example, a conventional zenon orother gas lamp which is known on the market as a lamp for producinghighly intense pulses of light. Certain tungsten and other lamps arealso useful.

The second apertured wall 6 forms an end wall for cavity 3 opposite endwall 7. A portion of the combined lasing-cooling structure 1 projectsinto the cavity 3 through aperture 11 in end wall 6. The combinedlasing-cooling structure 1 includes a rod 30 of active laser material.The laser material of rod 30 and the light producing electrode of lamp 2are conventionally placed at opposite focal points of the ellipticalcavity wall 5.

The combined lasing-cooling structure 1 is supported, for example, on abase plate 40 from a bracket 41 held in place by fastener 42. Bracket 41may take the form of means for clamping the combined lasing-coolingstructure 1 within a thermally insulating ring 43. The structure of theelliptical cavity 3 is cooperatively supported upon base 40 through theagency of fasteners 44 and 45.

One feature of the present invention is particularly concerned withadvantageous aspects of the combined lasingcooling structure 1. Thisstructure includes, as mentioned above, a rod 30 of active lasermaterial which may, for instance, be a rod of garnet that is doped witha particular ion whose presence fosters the stimulated amplification ofcoherent optical radiation. For the purpose of forming an opticalresonator, rod 30 is supplied with thin optically reflecting films 31,32 on its parallel-ground end faces. Film 31 is generally totallyreflecting, while film 32 is partially reflecting, permitting an intensebeam of coherent optical radiation to pass through the film 32 for useexternal of the inventive apparatus.

Rod 30 is confined, with its end film 31 substantially flush with theend 38 of tube 33, within the hollow optically refractive tube 33 (orsheath), which may be, for instance made of clear sapphire. Therelatively thick-walled tube or sheath 33 encompasses the completelength of rod 30 and extends well past the partially reflecting film 32and through aperture 1 1 of end wall 6 on out of cavity 3 to point 34.The phrase thick walled is used here to mean that the thickness of thetube wall is in the order of the diameter of the bore in tube 33.

The active laser rod 30 may be cemented within tubular sheath 33 byemploying a suitable transparent cement. Any of several materials areuseful for this purpose, including Canada balsam and clear epoxy. Sinceall of the parts of composite structure I operate at a very lowtemperature, ordinary viscous fluids are satisfactory as long as they donot absorb pumping radiation from lamp 2. For instance, tubular sheath33 may be telescoped over rod 30 after the cylindrical surface of rod 30is coated with such a viscous fluid. In operation at cryogenictemperatures, the viscous fluid becomes a natural solid, bonding theparts 30 and 34 together. Adhesives which form transparent solids mayalso be employed.

I Alternatively, the portion of the composite structure 1 including theactive medium of rod 30 and the transparent tubular sheath 33 may beformed by a conventional crystal drawing process. For example, thecomposite structure including rod 30 and sheath 33 may be made by firstpreparing the core rod 30 by making a single crystal of garnet orsimilar material, grinding, polishing, and coating its ends 31 and 32,and then using the crystal rod as a seed on which to grow a tube 33 ofsuitable material.

The composite structure 1 may be completed by coating onto the exteriorcylindrical surface of the end 34 of sapphire tube 33 a suitable heattransferring cement and then telescoping over tube 33 a drilled-outcopper cylinder 35. The accessible ends of the borders between sapphiretube 33 and copper cylinder 35 may be coated with any suitable epoxycement further to hold copper block 35 in place.

As noted previously, the active lasing material of rod 30 is supportedvia sapphire tube 33 and the copper cylinder 35 from base 40 by bracket41. The copper cylinder 35 is also supplied with a multiplicity of turnsof copper tubing 36 sol dered on its outer wall, through which turns anysuitable cryogenic cooling material may be circulated. Liquid nitrogenis an example ofa useful coolant.

The lasing material of rod 30 is thus cooled by thermal conduction ofheat from its surface through the volume of the sapphire tube 33. Heatthen flows into the copper cylinder 35 and is removed from the equipmentby the coolant flowing in copper tubing 36. The relatively thick-wallednature of sapphire tube 33 aids in focussing light from the pump lamp 2on the laser rod 30 and correspondingly aids in the efficient removal ofheat from it.

In the invention, an important feature of the means of cooling dependsupon recognition of the singular properties of the crystalline sapphirematerial of tube 33. First, it has the important property that it istransparent to electromagnetic radiation over a relatively wide range;that is, it is transparent at substantially all wave lengths at whichthe pumping lamp 2 radiates. Such transparency permits substantially allof the radiant energy from the pumping lamp 2 to reach laser rod 30.

In addition, the coefficient of thermal conductivity of sapphire isquite large. For example, it is considerably larger than brass and is,in fact, larger than copper over certain cryogenic ranges, such as from30 to 80 Kelvin. By way of example, the thermal conductivity of sapphirereaches 60 watts per centimeter degrees Kelvin near 40 Kelvin.

It is known that the thermally conductivity ofsapphire is not isotropic;in fact, conductivity perpendicular to the c-axis of the crystal is onthe order of 100 times smaller than the conductivity at 35 to thec-axis. Similar high thermal conductivity is achieved with use ofconduction parallel to the crystalline caxis. Accordingly, it ispreferred to place the c-axis of the crystalline sapphire rod 30substantially parallel to the axis of the composite structure 1.

While some of the undesired thermal energy appearing within the activelasing medium of rod 30 does flow radially out of the surface 37 of thesapphire tube 33 and this may contribute something to cooling the lasingmaterial of rod 30, exchange of heat into a gas or into a vacuumenvironment across the surface 37 of the sapphire material is relativelypoor. On the other hand, at temperatures desirable for the operation ofcertain lasing materials, the longitudinal path provided by a properlyoriented transparent sapphire tube or sheath 33 out of cavity 3 is ahighly efficient path for thermal flow especially if, as is practiced inthe present invention, the portion of sapphire tube 33 external ofcavity 3 is intensely cool.

For example, the present invention may also be applied to facilitatingefficient operation of a laser employing any one of a number of suitablegarnet materials, such as yttrium aluminum garnet doped with holmium orsimilar garnet materials doped with erbium. Several such materials laseefficiently at low temperatures such as 77 Kelvin but offer littlepotential at higher temperatures. The novel method of cooling of thepresent invention provides a significant improvement because elements ofthe cooling system now occupy a considerably reduced volume within thelaser pump cavity 3. Consequently. the laser pump cavity 3 may bedesirably reduced in size.

The efficiency of pumping action is correspondingly increased, as thesurface of wall 5 of the laser cavity 3 is reduced relative to thesurface area of the laser rod 30. Relatively more pumping energy is thenabsorbed in the laser rod 30, and relatively less in the wall 5 ofcavity 3. Pump efficiency may further be increased by opticallypolishing the pump cavity wall 5 and making it more highly reflecting byvacuum depositing a layer of gold upon the wall surface.

As noted previously, a feature of the invention lies in the fact that itenables the use of a practical and convenient configuration forcryogenic cooling of laser media in which parts primarily concerned withcooling may be located exterior of the pump cavity insofar as possible.FIG. 3 illustrates such a form of the invention, particularly showing anarrangement with minimum interference between pumping and coolingfunctions of the apparatus.

Referring to FIG. 3, it is observed that parts which correspond to partsfound in FIGS. 1 and 2 are identified by similar reference numerals; butin FIG. 3, a factor of has been added. Cooperation of these similarparts is similar to their cooperation in FIGS. 1 and 2, so that theirstructure and operation need not be explained here in detail. However,FIG. 3 illustrates an embodiment ofthe invention again including asprincipal elements a composite lasing and cooling system 101, a highintensity lamp 102 for stimulating lasing action, and an optical pumpingcavity 103 surrounding a portion of the composite lasing-cooling system101 and the pumping lamp 102. As before, the optical cavity 103 iselliptical in cross-section, and is defined by an enclosure or casing104 partly closed at its ends by apertured flat end walls 106 and 107.Aperture 108 in end wall 107 provides means for inserting within thecavity 103 the emission stimulating lamp 102, illustrated as mounted ina socket 109 and supplied with electrical power via input leads 125.

A second apertured wall 106 forms an end wall for cavity 103 oppositeend wall 107. The combined lasing-cooling configuration 101 projectsthrough aperture 111 in end wall 106 into the cavity 103 and includes arod of active laser material. The laser rod 130 and lamp 102 are placedat opposite focal lines of elliptical wall 105.

As previously discussed with reference to FIGS. 1 and 2, the compositelasing-cooling configuration 101 includes a rod 130 of active lasingmaterial supplied with thin optically reflecting films 131, 132 on itsparallel-ground end faces. Film 131 may be totally reflecting, whilefilm 132 is partially reflecting, permitting a beam of stimulatedcoherent radiation to pass through the film 132 for use external of theinvention.

The lasing rod 130 is suitably supported within a hollow tubular sheath133 of optically refractive material such as sapphire. The relativelythick-walled sheath 133 encompasses the complete length of rod 130 andextends well past the partially reflecting film 132 through aperture 111of end wall 106 and out of cavity 103 to an end point 134. The tubularsheath 133 is preferably made of clear sapphire cut with its crystallinecaxis substantially parallel to the longitudinal axis of the compositelasing-cooling configuration 101.

Inn. a...

The composite configuration 101 is completed by the drilled-out coppercylinder 135 telescoped over the end 134 of tubular sheath 133. As notedpreviously, the active lasing material of rod 130 is supported viasapphire sheath or tube 133 and the copper cylinder 135. Copper cylinder135 is supplied with a multiplicity of turns of copper tubing 136through which any suitable cryogenic cooling material may be circulatedvia respective input and output tubes 200, 201.

The lasing material of rod 130 is thus cryogenically cooled by thermalconduction of heat from its surface through the volume of the sapphiresheath 133. Heat then flows into the copper cylinder 135 and is removedfrom the equipment by the cryogenic coolant flowing in copper tubing200, 136, 201. If desired, a coolant such as water may be passed throughcopper tubes 203 wound about and soldered to the exterior surface ofcasing 104.

Certain elements of the invention yet to be described in connection withFIG. 3 play a primary role in supporting the principal elements of theinvention in an evacuated environment so as to prevent objectionableformation of condensate upon those principal elements. Alternatively, aninert gas at a near vacuum pressure level that does not significantlyabsorb energy from the radiation of lamp 102 and that is not itselfcondensable at the operating temperature of laser rod 130 may beemployed as an operating environment.

Referring now particularly to lamp 102 and socket 109, socket 109 issupported in a cylindrical region 204 having an apertured end wall 205and a cylindrical wall portion 206. Wall 206 has a radially extendingannular flange 207 at its inner end which may be fastened by anywell-known means for perfecting a vacuum tight joint 208 between flange207 and wall 107.

Wall 205 is provided with an aperture 209 for facilitating mounting ofsocket 109 and for permitting its withdrawal from cavity 103 wheninspection or replacement of lamp 102 is desired. Socket 109, which maybe water cooled 'by conventional means, is supported on a base plate 210held against a suitable vacuum gasket 211 by clamps 212 and 212 urgedagainst base plate 210 by screws 213 and 213. In the well known manner,a vacuum tight or hermetic seal is formed when screws 213, 213' aretightened, causing gasket 211 to form intimate hermetic or vacuum sealsat its annular contact with end wall 205 at its annular contact withbase plate 210. Evacuation of the interior of the structure, especiallyits interior in the vicinity of cavity 103 and volume 204, may beadditionally faciliated by attaching suitable vacuum pumping equipment(not shown) to an exhaust tube 214 which is hermetically sealed throughwall 206.

A second portion of the vacuum enclosure cooperates with wall 106 andforms means particularly for supporting the composite laser-coolingstructure 101. This supports structure includes a thin walled cylinder300, which may be composed of stainless steel or of any material havingsuch a low heat conductivity. Cylindrical wall 300 is provided at oneend with a radially extending flange 301 suitable for fastening to endwall 106 by brazing or by other known means for forming a vacuum tightseal at juncture 310. At the end of cylinder 300 opposite flange 301 isprovided a similar radially extending flange 302.

Flange 302 acts to support within cylinder 300 a second butsmaller-indiameter cylindrical wall 303, also made of thin walledstainless steel. Cylindrical wall 303 is provided with an inwardlyextending apertured plate or flange 304, also of thin stainless steel.Flange 304 is vacuum or hermetically sealed at the boundary 305 betweenit and metal block 135.

To facilitate mounting cylindrical wall 303 within cylindrical wall 300,wall 303 is provided with an outwardly extending flange 306. Flanges 302and 306 are adapted to form a vacuum tight or hermetic seal. For thispurpose, screws 308, 308 cooperate with flanges 302 and 306. When screws308, 308' are tightened, the annular gasket 307 between them is causedto form a hermetic or vacuum seal between itself and flange 302 and asecond such seal between itself and flange 306 in the well known manner.

The composite laser-cooling structure 101 is thus supported within adouble walled cylindrical configuration so that the active laser rod 130is accurately positioned at a focal line of cavity 103. Furthermore, thecomposite structure 101 is supported in a volume 410 whose evacuationmay be aided by pumping any atmosphere it contains by well-known vacuumpump means (not shown) attached to an exhaust tube 309 which perforateswall 300 and is vacuum sealed therein. A further function of thecomposite support-envelope system is that the coaxially arranged thincylindrical walls 300 and 303 together form a long and relatively poorheat conduction path. Thus, the composite laser-cooling structure 101 issubstantially thermally isolated from the wall or casing 105 of cavity103.

Radiation from the lasing material passes through the hollow spacewithin tube 133 and out past its end 134. Since the high intensityradiation must be permitted to flow out of the apparatus for externaluse, a suitable transparent window 400 is supplied adjacent end 134 oftube 133. Window 400 may be supported within an apertured disk 401,being sealed thereto by any known vacuum tight sealing method. Disk 401is, in turn, sealed at junction 402 to a radially extending flange 403located at the window end of the thin walled stainless steel tube 404.Cylindrical wall 404 is, in turn, provided with a radial flange 405hermetically sealed to metal block 135.

Cylindrical wall 404 is seen to provide support means for window 400 andalso, together with window 400 and plate 401, to complete the vacuumenvelope about volume 411 of the apparatus. Because cylindrical wall 404is made of stainless steel and is relatively long, it forms a poor heatpath from metal block 135 toward window 400; thus, both sides of window400 are maintained relatively free of condensate during the operation ofthe apparatus.

An output exhaust tube (not shown) may be sealed within cylindrical wall404 to permit evacuation of its contents from volume 411. Alternatively,one or more channels 406 and 406' may be drilled within metal block 135in such a manner as directly to connect all volumes to be evacuated bytubes 214 or 309 with the volume 411 sealed within cylindrical wall 404.

[t is thus seen that means have been provided in FIG. 3 for permittingthe principal elements of the invention, as illustrated in FIGS. 1 and2, to be placed in an evacuated environment so as to prevent formationof condensate upon those principal elements, which condensate mightotherwise form and degrade operation of the invention because of the lowoperating temperature of the combined laser-cooling structure 101. It isseen that the additional elements described in connection with FIG. 3have added roles to play, including mechanical support of primaryelements such as lamp 102, lasing-cooling configuration 101, and window400. They also serve as low thermal conductivity paths, preventingundesired flow of heat along the respective mechanical supportstructures and into the configuration 101.

Furthermore, it is seen that the arrangement disclosed in FIG. 3 permitsready removal of lamp 102 and its substitution by a similar lamp or by alamp having a different radiation characteristic. Also permitted is theremoval of the combined laser-cooling configuration 101 for the purposeof inspection or of replacement of rod 130 with a similar rod in theevent it is found to be damaged, or permitting its replacement with arod of a different lasing material, if desired. It is within the scopeof the invention to replace demountable vacuum joints, for example, at211 or 307 with permanently sealed joints, or to substitute anypermanently sealed joint with a demountable vacuum joint, as desired.

From the foregoing, it is clear that the inventive concept providesmeans for overcoming the serious deficiencies of the prior art inproviding a novel composite lasing and cooling configuration whereinheat generated within a lasing medium is efficiently and effectivelyremoved from the medium and is transferred to a heat sink remote fromthe laser medium and its pump cavity. The composite lasing and coolingsystem employs a tube or sheath surrounding the lasing medium forperforming the heat transfer function.

As has been described, several beneficial properties of the material ofthe tube or sheath are exploited by the invention. In particular, theselected material is transparent to substantially all wave lengths ofelectromagnetic energy radiated by the pump lamp and the sheath actsefiiciently to focus substantially all of that radiated energy upon thelaser material. Substantially none of the pump lamp energy is absorbeddirectly within the sheath, so that substantially all of the pump energyreaches the desired site and therefore cannot itself raise thetemperature of the sheath and, more important, of the contiguous lasingmaterial to an undesirable extent. Selection of an ideal material forthe sheath is important to the success of the invention, since anyenergy that might be absorbed by the sheath would also have to bedissipated. The presence of any such extra heat source would effectivelyresult in an increased temperature gradient between the laser medium andthe external heat sink, and is avoided by the inventive composite lasingand cooling system. Further, a sheath oriented as described above withrespect to its c-axis forms an efficient heat path from the lasingmaterial to the heat sink, and has other mechanical and opticalproperties ideally compatible with the material parts with which itcooperates. While sapphire has been used in the foregoing discussion asone practical example of a suitable material for the sheath, any othermaterial having similar beneficial properties may be employed.

While the invention has been described in its preferred embodiment, itis to be understood that the words which have been used are words ofdescription rather than limitation and that changes may be made withoutdeparture from the true scope and spirit of the invention in its broaderaspects.

I claim 1. In combination:

an elongate body of lasable material having a substantially cylindricalsurface and first and second ends,

said lasable body having a length between said ends substantiallygreater than the transverse dimension of said body,

reflective means at said ends for forming an energy radiating resonantcavity about said lasable body,

an elongate homogeneous body of material relatively transparent in thewave length range suitable for pumping said lasable body, saidtransparent body taking the form of a hollow tube having substantiallycoaxial inner and outer surfaces, said cylindrical surface of saidlasable body being disposed in intimate contact with said inner surfaceof said transparent hollow tube for efficient transfer of heat from saidlasable body to said hollow tube over substantially the total area ofsaid cylindrical surface of said lasable body, an elongate metal body ofrelatively high heat conductivity. said body taking the form ofa hollowmetal element having a substantially cylindrical inner surface, saidouter surface of said transparent hollow tube being disposed in intimatecontact with said inner surface of said metal body for efficienttransfer of heat from said transparent hollow tube to said metal bodyover substantially the total area of said cylindrical inner surface ofsaid metal body, means for cooling said metal body, said metal bodybeing spaced on said transparent hollow tube from said lasable body innon-overlapping relation therewith, means for projecting pumping energythrough said body of transparent material for substantially uniformlyilluminating the total of said cylindrical surface of said lasablematerial, and wherein said elongate tubular body of transparent materialhas an effective direction of heat conductivity lying substantiallyparallel to said inner and outer coaxial surface of said elongate hollowtubular body for efiicient transfer of heat from said lasable body tosaid metal body. 2. Apparatus as described in claim 1 wherein saidelongate tubular body of transparent material is comprised ofcrystalline sapphire having its most effective direction of heatconductivity lying substantially parallel to said inner and outercoaxial surfaces of said elongate hollow tubular body for efficienttransfer of heat from said lasable body to said metal body.

1. In combination: an elongate body of lasable material having asubstantially cylindrical surface and first and second ends, saidlasable body having a length between said ends substantially greaterthan the transverse dimension of said body, reflective means at saidends for forming an energy radiating resonant cavity about said lasablebody, an elongate homogeneous body of material relatively transparent inthe wave length range suitable for pumping said lasable body, saidtransparent body taking the form of a hollow tube having substantiallycoaxial inner and outer surfaces, said cylindrical surface of saidlasable body being disposed in intimate contact with said inner surfaceof said transparent hollow tube for efficient transfer of heat from saidlasable body to said hollow tube over substantially the total area ofsaid cylindrical surface of said lasable body, an elongate metal body ofrelatively high heat conductivity, said body taking the form of a hollowmetal element having a substantially cylindrical inner surface, saidouter surface of said transparent hollow tube being disposed in intimatecontact with said inner surface of said metal body for efficienttransfer of heat from said transparent hollow tube to said metal bodyover substantially the total area of said cylindrical inner surface ofsaid metal body, means for cooling said metal body, said metal bodybeing spaced on said transparent hollow tube from said lasable body innon-overlapping relation therewith, means for projecting pumping energythrough said body of transparent material for substantially uniformlyilluminating the total of said cylindrical surface of said lasablematerial, and wherein said elongate tubular body of transparent materialhas an effective direction of heat conductivity lying substantiallyparallel to said inner and outer coaxial surface of said elongate hollowtubular body for efficient transfer of heat from said lasable body tosaid metal body.
 2. Apparatus as described in claim 1 wherein saidelongate tubular body of transparent material is comprised ofcrystalline sapphire having its most effective direction of heatconductivity lying substantially parallel to said inner and outercoaxial surfaces of said elongate hollow tubular body for efficienttransfer of heat from said lasable body to said metal body.